1. Field of the Invention
The present invention relates to an electron-beam lithographic method and, more particularly, to an electron-beam lithographic method for writing a desired pattern on a material by passing an electron beam through the aperture in a first slit (apertured baffle plate) and then directing the beam at a desired position on a second slit (apertured baffle plate) to shape the cross section of the beam into a desired shape, deflecting this deflected beam, and directing the beam at a desired location on the material.
2. Description of Related Art
In a variable-area electron-beam lithography machine, an IC (integrated circuit) pattern is formed on a material to be written, such as a mask plate, by combining various graphical figures, such as rectangular, square, triangular, and trapezoidal forms. In a lithography machine using an electron beam having a rectangular cross section with four right-angled corners, every pattern having oblique lines, such as a triangular pattern, is approximately resolved into rectangles and then lithography is carried out. This method is simple but has the disadvantage that the writing speed is slow.
In recent years, a novel electron-beam lithography machine making use of an electron beam having a non-rectangular cross section has been developed in order to perform lithography at a higher speed. In particular, with respect to a pattern having forms often encountered in LSI (large scale integration) circuits, such as a pattern having 45-degree oblique lines, various apertured baffle plates are prepared which are capable of shaping the cross section of the beam into such forms. That is, the cross section of the beam is shaped into a form different from a rectangle with four right-angled corners.
Where a pattern is created using an electron beam of a cross section having four right-angled isosceles triangles oriented in different directions and a rectangle with four right-angled corners, if the written pattern consists only of 45-degree oblique lines, then it is simple to determine how these portions of the cross section of the beam are used. However, in LSI circuit design, it is advantageous to be able to use data about any arbitrary angle. Therefore, it is unrealistic that the oblique lines of a pattern to be written are only 45-degree oblique lines.
In order to make effective use of the aforementioned electron beam having a cross section of right-angled isosceles triangles to provide improved writing speed, it is necessary to extract data about the right-angled isosceles triangles found in a pattern from data about the pattern to be written.
A high efficiency is not obtained where the conventional division algorithm is employed which uses a beam having a rectangular cross section. For example, where a pattern having a shape as indicated by FIG. 1A is written, if the width of the pattern is smaller than the lateral dimension of the rectangular cross section of an electron beam, the whole pattern is divided into portions as indicated by FIG. 1B. Then, the portions including oblique lines are split into stripes and written as indicated by FIG. 1C.
On the other hand, where the width of the pattern is greater than the lateral dimension of the rectangular cross section of the beam as indicated by FIG. 1Axe2x80x2, the pattern is vertically divided into parts from the oblique lines as indicated by FIG. 1D. The oblique-side portions of the split portions are separated as triangular portions. These separated triangular portions are split into stripes and written as indicated by FIG. 1E.
In the case of FIG. 1D, since the triangular portions are separated, if the angle of the oblique-side portions is 45 degrees, then it is easy to use the electron beam having a cross section of right-angled isosceles triangles. However, in the case of FIG. 1B, even if the angle of the oblique-side portions is 45 degrees, there is no chance of using the electron beam having a cross section of right-angled isosceles triangles, because the triangular portions cannot be separated.
It is an object of the present invention to provide a lithographic method which provides an improved writing speed by the use of a variable-area electron-beam lithography machine by making effective use of an electron beam having a cross section shaped into right-angled isosceles triangles.
A lithographic method according to the present invention uses a variable-area electron-beam lithography machine having first and second apertured baffle plates, each of the apertured baffle plates having an aperture. The lithographic method comprises the steps of: passing an electron beam through the first baffle plate having the aperture with four right-angled corners; directing the beam emerging from the aperture in the first baffle plate at the second baffle plate; and directing the beam emerging from the aperture in the second baffle plate at a material to be written. The electron-beam lithography machine is capable of shaping the cross section of the beam emerging from the aperture in the second baffle plate into a rectangle or right-angled isosceles triangles. Where a trapezoidal graphical figure having at least one leg (oblique side) making an angle of 45 degrees with respect to the normal to the bottom base is to be written on the material, a division algorithm for extracting (isolating) a portion including the 45-degree leg is used. The extracted portion including the leg is written by the right-angled isosceles triangles defined by the cross section of the beam.
Another lithographic method according to the present invention uses a variable-area electron-beam lithography machine as described. When a trapezoidal graphical figure is to be written on the material, a sequence of operations is performed. This sequence of operations consists of performing a first step consisting of making a decision as to whether the angle made between one leg (oblique side) of the trapezoidal figure and the normal to the bottom base is 45 degrees and whether the angle made between the other leg and the normal to the bottom base is 45 or 0 degrees, performing a second step consisting of making a decision as to whether it is necessary to divide the trapezoidal graphical figure in height direction (that is, in the direction perpendicular to the base) if the result of the decision made in the first step is YES, performing a third step consisting of dividing the trapezoidal graphical figure in the height direction using a unit division length equal to the length of the bottom base of the trapezoidal graphical figure, separating portions including the legs from the resulting graphical figure parts, and writing the separated portions by the right-angled isosceles triangles of the cross section of the beam if the result of the decision made in the second step is YES, performing a fourth step consisting of separating the portions including the legs of the trapezoidal graphical figure and writing the separated portions by the right-angled isosceles triangles of the cross section of the beam if the result of the decision made in the second step is NO, performing a fifth step consisting of making a decision as to whether it is necessary to separate the portions including the legs if the result of the decision made in the first step is NO, and performing a sixth step consisting of separating the portions of the legs if the result of the decision made in the fifth step is YES and writing the portions including the legs by the right-angled isosceles triangles of the cross section of the beam if the angle made between the leg of each separated portion and the bottom base is 45 degrees.
In a further lithographic method according to the present invention uses a variable-area electron-beam lithography machine having, as described, a trapezoidal graphical figure to be written on the material, a sequence of operations is performed. This sequence of operations consists of performing a first step consisting of making a decision as to whether the angle made between one leg of the trapezoidal graphical figure and the normal to the bottom base is 45 degrees and whether the angle made between the other leg and the normal to the bottom base is 45 degrees, performing a second step consisting of making a decision as to whether it is necessary to divide the trapezoidal graphical figure in height direction (that is, in the direction perpendicular to the base) if the result of the decision made in the first step is YES, performing a third step consisting of dividing the trapezoidal graphical figure in the height direction using a unit division length equal to the length of the bottom base of the trapezoidal graphical figure, separating portions including the legs from the resulting graphical figure parts, and writing the separated portions by the right-angled isosceles triangles of the cross section of the beam if the result of the decision made in the second step is YES, performing a fourth step consisting of separating the portions including the legs of the trapezoidal graphical figure and writing the separated portions by the right-angled isosceles triangles of the cross section of the beam if the result of the decision made in the second step is NO, performing a fifth step consisting of making a decision as to whether it is necessary to separate the portions including the legs if the result of the decision made in the first step is NO, and performing a sixth step consisting of separating the portions including the legs if the result of the decision made in the fifth step is YES and writing the portions including the legs by the right-angled isosceles triangles of the cross section of the beam if the angle made between the leg of each separated portion and the bottom base is 45 degrees.
Still another lithographic method according to the present invention uses a variable-area electron-beam lithography machine as described where a trapezoidal graphical figure is to be written on the material, a sequence of operations is performed. This sequence of operations consists of performing a first step consisting of making a decision as to whether the angle made between any one leg of the trapezoidal graphical figure and the normal to the bottom base is 45 degrees, performing a second step consisting of making a decision as to whether it is necessary to divide the trapezoidal graphical figure if the result of the decision made in the first step is YES, performing a third step consisting of dividing the trapezoidal graphical figure in height direction (that is, in the direction perpendicular to the base) using a unit division length equal to the length of the bottom base of the trapezoidal graphical figure, separating portions including the legs from the resulting graphical figure parts, and writing the separated portions by the right-angled isosceles triangles of the cross section of the beam if the result of the decision made in the second step is YES, performing a fourth step consisting of separating the portions including the legs of the trapezoidal graphical figure and writing the separated portions by the right-angled isosceles triangles of the cross section of the beam if the result of the decision made in the second step is NO, performing a fifth step consisting of making a decision as to whether it is necessary to separate portions including the legs if the result of the decision made in the first step is NO, and performing a sixth step consisting of separating the portions including the legs if the result of the decision made in the fifth step is YES and writing the portions including the legs by the right-angled isosceles triangles of the cross section of the beam if the angle made between the leg of each separated portion and the bottom base is 45 degrees.
Yet another lithographic method according to the present invention uses a variable-area electron-beam lithography machine as described where a written graphical figure is a parallelogram including a top base, a bottom base, and two legs, if each of the legs makes an angle of 45 degrees with respect to the normal to the bottom base, and if the bottom base is shorter than the length of two mutually perpendicular sides of a shapeable maximum right-angled isosceles triangle of the cross section of the electron beam, then a division algorithm for dividing the parallelogram vertically with a unit division length equal to the length of the bottom base of the parallelogram is used and graphical figure portions obtained by the division are written by the right-angled isosceles triangles or a combination of the right-angled isosceles triangles and the rectangle with four right-angled corners of the cross section of the beam.
Other objects and features of the invention will appear in the course of the description thereof, which follows.